In a modern aircraft the passenger cabin or sub-regions of the passenger cabin, the cargo hold or sub-regions of the cargo hold and the cockpit form various air conditioning zones that are conventionally air-conditioned by means of an aircraft air conditioning system both during cruising and during taxiing of the aircraft. Hot bleed air under an elevated pressure is removed from the engine compressors or auxiliary engine compressors and supplied to the aircraft air conditioning system. In the air conditioning units, the so-called air conditioning packs of the aircraft air conditioning system, the bleed air is expanded and cooled down to a desired low temperature. Finally, the air conditioned in the air conditioning units is conveyed as cooled fresh air into a main fresh-air line of the air conditioning system. The fresh air flowing through the main fresh-air line is conveyed into a mixing device, where it is mixed with recirculation air that is extracted from the passenger cabin. The mixed air produced in the mixing device from cold fresh air provided by the air conditioning packs and recirculation air extracted from the passenger cabin is finally used to air-condition the various air conditioning zones of the aircraft.
Mixing devices currently used in aircraft air conditioning systems are connected by appropriate connections to a plurality of pipe systems, through which, depending on the operating state of the aircraft, air flows at different flow rates and at different temperatures. However, the arrangement, the geometry and the course of the connections and pipe systems may be optimized usually only for one operating state, i.e. one flow rate and one temperature. Installation space restrictions on board the aircraft moreover influence the geometry and the course of the connections and pipe systems. In dependence upon the flow rate and the temperature of the air supplied to the mixing device through the pipe systems temperature differences within the mixing devices and hence variations of the mixing quality may therefore arise, wherein in particular a fault scenario linked to failure of an air flow may considerably impair the mixing quality.
Insufficient mixing of the cold and warm air flows in the mixing device makes it difficult or even impossible to supply all of the air outlets of the mixing device with air at the desired same temperature and with the desired same fresh-air/recirculation-air ratio. This may result in comfort losses, but also in a non-uniform supply of fresh air to the persons situated on board the aircraft. Furthermore, in operating situations of the air conditioning system, in which the cold fresh air is supplied to the mixing device at a temperature <0° C., inadequate mixing of the cold fresh air supplied to the mixing device with warm recirculation air may lead, because of the moisture contained in the fresh air, to the attachment of ice and snow in the mixing device or in the pipe systems connected to the mixing devices.
In order to be able to guarantee a satisfactory mixing quality, mixing devices currently used in aircraft air conditioning systems need to have as great a mixing path length as possible, with the result that the mixing devices are relatively bulky. In the mixing devices pressure losses moreover occur, which have to be compensated by a corresponding high conveying capacity of conveying devices used to convey the air flows through the mixing devices.
The invention is geared to the object of indicating a mixing device for an aircraft air conditioning system that enables a uniform mixing of the warm and cold air flows that are supplied to the mixing device. The invention is further geared to the object of providing an aircraft air conditioning system equipped with such a mixing device.
This object is achieved by a mixing device for an aircraft air conditioning system having the features of attached claims and by an aircraft air conditioning system having the features of attached claims.
A mixing device suitable for use in an aircraft air conditioning system comprises a plurality of cold-air supply lines, which are adapted to supply cold air to the mixing device. The cold air to be supplied to the mixing device may be produced for example by an air conditioning unit or a plurality of air conditioning units of the air conditioning system. For example, two cold-air supply lines may be provided, which connect the mixing device to two air conditioning units of the aircraft air conditioning system that are constructed separately from one another. The mixing device according to the invention further comprises a plurality of warm-air supply lines, which are adapted to supply warm air to the mixing device. The warm-air supply lines are preferably recirculation-air lines, which connect the mixing device to a passenger cabin or a cargo hold of the aircraft and convey recirculation air from the passenger cabin or cargo hold into the mixing device. The mixing device according to the invention preferably comprises at least two warm-air supply lines, which connect the mixing device for example to various regions of the passenger cabin and/or of the cargo hold.
A first premixing chamber of the mixing device is connected to the cold-air supply lines and adapted to mix the cold air that is fed through the cold-air supply lines into the first admixing chamber. A second premixing chamber is further provided, which is connected to the warm-air supply lines and adapted to mix the warm air that is fed through the warm-air supply lines into the second premixing chamber. In the first premixing chamber, therefore, a premixing of the cold air fed through the cold-air supply lines to the mixing device occurs before the cold air comes into contact with the warm air fed through the warm-air supply lines. In a similar fashion, the second premixing chamber is used to premix the warm air fed through the warm-air supply lines to the mixing device before the warm air comes into contact with the cold air fed through the cold-air supply lines to the mixing device.
The mixing device according to the invention further comprises a main mixing chamber, which is connected to the first and the second premixing chamber and devised to mix the cold air premixed in the first premixing chamber with the warm air premixed in the second premixing chamber. The main mixing chamber of the mixing device according to the invention is preferably supplied with premixed air exclusively by means of the premixing chambers, i.e. a direct supply of non-premixed cold or warm air into the main mixing chamber is not provided. The mixing device according to the invention is preferably consequently as a whole of a two-stage construction, i.e. all of the air flows supplied to the mixing device are first treated in one of the premixing chambers before being supplied to the main mixing chamber.
The mixing device according to the invention is notable for the fact that independently of the operating situation of the aircraft air conditioning system, i.e. independently of variations of the flow rates and/or temperatures of the air flows supplied to the mixing device, a constant mixing quantity may be achieved. This applies even in a fault scenario linked to failure of an air flow, i.e. with the mixing device according to the invention even in the case of such a fault scenario it is still possible to realize a satisfactory mixing quality. A further advantage of the mixing device according to the invention is that by virtue of the uniform mixing of the cold air supplied to the mixing device with warm air it is possible to minimize an attachment of ice and snow in the mixing device or in the pipe systems connected to the mixing device that is caused by inadequate mixing of the air flows supplied to the mixing device.
The first premixing chamber of the mixing device according to the invention is preferably connected to a cold-air discharge line for discharging cold air from the first premixing chamber. Through the cold-air discharge line cold air that is not mixed with warm air may be removed from the mixing device and supplied for example as a fresh air supply to a cockpit of the aircraft. By virtue of the connection of the cold-air discharge line to the first premixing chamber it is guaranteed that even in the event of failure of the cold air supply through one of the cold-air supply lines connected to the first premixing chamber a discharge of cold air from the first premixing chamber and hence a supply of fresh air, for example to the cockpit of the aircraft, is possible.
Alternatively or additionally the second premixing chamber of the mixing device according to the invention may be connected to a warm-air discharge line for discharging warm air from the second premixing chamber. Warm air may be supplied through the warm-air discharge line to a system of the aircraft that requires warm air, for example to a de-icing system or the like. By virtue of the connection of the warm-air discharge line to the second premixing chamber it is ensured that, even in the event of failure of one of the warm air flows through one of the warm-air supply lines, enough warm air is available for discharge from the second premixing chamber through the warm-air discharge line.
In principle, the first premixing chamber and the second premixing chamber may be disposed in any desired manner relative to one another. For example, it is conceivable to dispose the second premixing chamber upstream of the first premixing chamber. In a preferred form of implementation of the mixing device according to the invention the first premixing chamber is however disposed upstream of the second premixing chamber. The first premixing chamber is preferably connected to the main mixing chamber via a nozzle that gives rise to an acceleration of the cold air flow conveyed from the first premixing chamber into the main mixing chamber. If the first premixing chamber is disposed upstream of the second premixing chamber, the nozzle connecting the first premixing chamber to the main mixing chamber preferably extends through the second premixing chamber. The nozzle preferably has at least in sections a flow cross section that widens in the direction of the cold air flow through the nozzle, i.e. is preferably at least in sections of a conical configuration.
The nozzle is preferably designed in such a way that at least in an inlet region of the nozzle facing the first premixing chamber a ratio between a generated surface of the nozzle and a through-flow volume of the nozzle increases in the direction of the cold air flow through the nozzle. An increase of the ratio between the generated surface and the through-flow volume of the nozzle and hence the air jet volume flowing through the nozzle enables an acceleration of the air jet flowing through the nozzle and hence the build-up of a velocity difference between the cold air flow, which is conveyed from the first premixing chamber into the main mixing chamber through the nozzle, and the warm air flow, which is not accelerated as it is fed from the second premixing chamber into the main mixing chamber. The cold air flow that is accelerated by means of the nozzle then acts as an injector, with the result that the pressure loss arising in the mixing device may be minimized. The injector effect of the nozzle moreover promotes mixing of the cold air exiting from the nozzle with the warm air flowing around the nozzle and therefore minimizes the required mixing path length. Consequently, the mixing device as a whole may be of a smaller design and therefore take up less installation space.
The nozzle is moreover preferably designed in such a way that in an outlet region, i.e. a region of the nozzle facing the main mixing chamber, the generated surface of the nozzle remains substantially constant in the direction of the cold air flow through the nozzle. Such a design of the nozzle prevents the occurrence of flow separation at divergent surfaces of the nozzle in the outlet region of the nozzle.
Finally, an outlet of the nozzle is preferably designed in such a way that a cold air flow exiting from the outlet of the nozzle forms a plurality of free jets. This optimizes the distribution of the cold air in the main mixing chamber and improves the mixing effect of the mixing device.
In an inlet region of the main mixing chamber facing the premixing chambers the main mixing chamber preferably comprises a diffuser. For example, the diffuser may lie downstream of, and adjacent to, the nozzle for supplying the cold air from the first premixing chamber into the main mixing chamber. Given such a configuration, the cold air from the nozzle is conveyed after exiting from the nozzle outlet into the diffuser, where an initial mixing with the warm air flowing around the nozzle may take place. Through the use of a diffuser in the main mixing chamber the mixing effect of the mixing device according to the invention may be further improved.
In the inlet region of the main mixing chamber facing the premixing chambers a flow control device may further be provided. For example, the flow control device may be disposed immediately downstream of the diffuser provided in the inlet region of the main mixing chamber. The flow control device is used preferably to guide the free jet that exits from the nozzle connecting the first premixing chamber to the main mixing chamber. In particular, the flow control device is used to correct unwanted deflections of the free jet that occur as a result of flow separation in the nozzle caused by extreme velocity differences between the cold air flow in the nozzle and the warm air flow surrounding the nozzle. Thus, by means of the flow control device an impairment of the mixing quality as a result of an unwanted deflection of the free jet exiting from the nozzle may be prevented.
The flow control device preferably comprises two flow baffle plates, which are curved in the direction of the cold air flow through the inlet region of the main mixing chamber and extend from mutually opposite side walls of the main mixing chamber into the cold air flow flowing through the inlet region of the main mixing chamber. The flow baffle plates of the flow control device are preferably configured in the form of triangular plates and disposed in such a way that in each case a point of the triangular plates extends from the mutually opposite side walls of the main mixing chamber into the cold air flow flowing through the inlet region of the main mixing chamber.
Finally, the mixing device according to the invention preferably comprises an anti-icing device. The anti-icing device may be disposed in the region of a rear wall of the main mixing chamber that lies opposite the inlet region of the main mixing chamber. By means of the anti-icing device ice may be prevented from depositing on the rear wall of the main mixing chamber lying opposite the inlet region of the main mixing chamber.
An aircraft air conditioning system according to the invention comprises a previously described mixing device.